Coastal Deposition

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Coastal Deposition

Coastal deposition describes the sea dropping sediment (or material) being carried by the water at the coastline. Coasts grow in size when more sediment is deposited on the coast than is lost to the process of erosion.

Why does deposition happen?

Deposition happens when constructive waves break on the shore. The sediment being carried by the sea is put (deposited) on the coastline.
This usually creates beaches made of sand or shingle in the area between the 'high' water mark and the 'low' water mark.
The high water mark is the point the highest up the beach that the sea level rises to.
The low water mark is the point the lowest down the beach that the sea level falls to.

Water marks

An image of high and low water marks.

What increases the amount of deposition?

If erosion on the nearby coast is high, then lots of rock and sand will fall into the ocean. This will increase deposition on nearby beaches.
The more sediment in the ocean, the more deposition there will be.

Why are some beaches sandy & some shingle?

Sandy beaches are created by sand being deposited on the shoreline.
Sandy beaches are usually very long, wide, and flat because particles of sand are very small and easy to wash back into the ocean with the backwash.
Shingle beaches are made when pebbles and shingle are deposited on the coast. Shingle and pebbles are big and hard to wash back into the ocean, so they build up (making short and steep beaches).

Landforms of Coastal Deposition

Beaches, spits, bars and sand dunes are all landforms of coastal deposition. Longshore drift is a key form of coastal transportation, which enables the creations of certain landforms of deposition.

Coastal deposition

Coastal deposition is when material is dropped by seawater on the shore. Coasts get built up when there is more deposition than erosion.
Deposition happens when low energy, constructive waves transport material onto the coast but don't have enough energy to remove it all.
High levels of erosion at other parts of the coast causes deposition. This means that there is lots of material to be transported and deposited.

Longshore drift

Longshore drift transports material along coastlines.
- Wind approaches the coast at an angle because of prevailing wind directions.
- Waves are controlled by wind and so this angle will be the direction the swash moves up the beach.
- Gravity is the only force that acts on the backwash, so it falls back to the sea at 90° to the coastline.
- This causes sediment to repeatedly move in the shape of a right-angled triangle. Over time, sediment is carried along a beach.

Bars

A bar is formed when a spit grows across a bay.
Lagoons often form behind bars.

Beaches

Beaches can be found between the high and low water mark.
Beaches are made by constructive waves moving rock fragments inland.
Sand beaches are flat and wide, with a long and gentle slope.
Shingle beaches are steep and narrow, with a short and steep slope.

Spits

When the coast changes direction, longshore drift continues to move sediment and deposit it.
Spits are the long fingers of sand sticking out from one side in a coastline. They often have salt marshes behind them.
If the direction of the wind changes they can form curved ends, known as recurved ends.